1. Field of the Invention
The invention relates generally to the field of nanotechnology. More particularly, the invention relates to cantilever structures that include a vertically aligned nanostructure and procedures for making a cantilever structure with a vertically aligned nanostructure. Specific embodiments of the invention are directed to scanning probe microscope tips and procedures for making scanning probe microscope tips.
2. Discussion of the Related Art
Scanning probe microscopy (SPM) has become an indispensable tool for the analysis of surfaces and the exploration of general substrate morphology at the nano- and microscale. Moreover, SPM techniques have been used to investigate a wide variety of material properties beyond substrate topography. These uses include the mapping of magnetic domains within a material (magnetic force microscopy, MFM), the mapping of chemical properties on a sample surface (chemical force microscopy, CFM), the electrochemical properties of surface (electrochemical microscopy, ECM) the manipulation of materials at the nanoscale and high-resolution nanolithography, among others. These applications span a vast array of scientific disciplines ranging from solid state physics to biology, environmental science to polymer engineering. High resolution SPM systems can be procured, operated and maintained at a fraction of the cost of other high resolution microscopy instruments (e.g. scanning electron and transmission electron microscopes) making it, perhaps, one of the most cost effective scientific tools available to the modern researcher.
High aspect ratio SPM tips are necessary to accurately characterize microscopic surfaces with nanoscale features. It has long been understood that the tip geometry of the scanning probe becomes convolved with the sample topography during imaging [1,2]. This can generate several artifacts that obscure important features of a samples' surface structure. Micro- and nanofabricated structures as well as complex biological systems posses a varied surface topography and can only be imaged effectively using this type of tip. Moreover, a small tip cross section ensures negligible variations in contact radius despite wearing of the tip's end during scanning and reduces the interaction force between tip and sample.
There are currently three methods used for producing high aspect ratio tips for SPM applications: ultrasharp Si or W tips [2–6], focused electron beam deposited amorphous carbon tips [7–9] and carbon nanotube (CNT) tips [10–22]. While all three of these types of tips offer significant improvements over traditional etched Si tips, each suffer from several key disadvantages which limit their usefulness and/or their ability to be produced in a large scale process. For example, ultrasharp Si tips are typically produced using focused ion beam milling. This is a serial process capable of only fabricating a single tip at a time. While this process can be automated with a high degree of precision it is time consuming and results in a tip that is very brittle that degrades easily. Focused electron beam deposited amorphous carbon tips are more mechanically robust than high aspect ratio Si tips. Unfortunately, the fabrication process is also serial in nature; only a single tip can be fabricated at a time.
CNT-based tips have been touted as the ultimate solution to the problem of manufacturing high aspect ratio SPM tips. Single walled CNT material has a diameter on the order of 1 nm and can be up to hundreds of microns long producing a structure that is intrinsically very high aspect ratio. Much work has been done on the integration of CNT growth with Si SPM tips to form hybrid supertips. Dai et al have demonstrated the ability to produce these tips on a wafer scale [21]. However, the control over the length and angle of the CNT protrusion from the Si tip is a significant problem. In a recent work by Snow et al it was shown theoretically and experimentally that CNT tips produced in this manner do not represent a significant improvement in imaging performance over other types of high aspect ratio tips [22]. They found that nonvertical alignment of the CNT tip with respect to the cantilever body caused the probe to bend in response to the surface-nanotube interaction forces during imaging. For long nanotubes, this elastic response rendered the tips unsuitable for imaging. For short nanotubes stable imaging was achieved using a small cantilever vibration amplitude. However, the bending response was found to be enhanced on highly textured surfaces limiting the ability to image nonplanar features. While a method by Dai has been demonstrated for shortening CNT tips to a suitably short length for controllable imaging this reduces their production to a serial process. It should also be noted that this offers no way to correct for variations in the alignment of the CNT with respect to the supporting Si tip. These facts combined with the results reported by Snow clearly shows that CNT-based SPM tips are not the ideal solution to this problem. A means of producing high aspect ratio tips that can simultaneously meet the following criteria is desperately needed: it must be mechanically robust, composed of a wear resistant material, created at a controlled angle with respect to the cantilever structure that it is attached to and be integrated into a wafer scale batch fabrication process.
Catalytically grown carbon fibers have been investigated for decades beginning in 1950's [23]. Chen et al. showed that carbon nanofibers (CNF) could be aligned perpendicular to the substrate in a direct current (DC) plasma enhanced chemical vapor deposition (PECVD) process [24]. Since that time a number of studies on the growth and properties of vertically aligned CNF (VACNF) have been conducted.